1. Technical Field
The present invention relates to a method and apparatus for centering a log.
2. Background Art
When a log is peeled on a veneer lathe, it is necessary to accurately determine the position of optimum yield axis in order to improve the rate at which continuous veneer sheets can be obtained, or the yield of veneer sheets. For this purpose, it is considered effective and practical to rotate the log about a preliminary axis and then measure the contour of the log at, preferably, at least the vicinity of either end thereof or, more preferably, at the vicinity of a center portion of the log as well as the ends thereof. If necessary, such as when the log is of a great length exceeding 2 m, the contour is also measured at an intermediate portion between each end and the center portion (namely, at one location each towards the left and right of the log). Then, based on the resultant contour data (shapes, relative positional relationships, etc.), a desired optimum yield axis is calculated.
Further, in order to reduce the time in which the log is idling, while avoiding the collision between the log and the knife carriage, it is necessary to determine the maximum radius of rotation of the log and set the knife carriage at a proper standby position each time the log is peeled. It would be convenient if the maximum radius of rotation of the log can be determined simultaneously with the position of the optimum yield axis. However, the maximum radius of rotation of the log cannot be determined unless the optimum yield axis is set. Further, if the contour of the log is not measured at a sufficiently large number of points along the axis of the log (i.e., if there is too much area that is not measured), the log could collide with the knife carriage. Therefore, the contour of the log must be measured at a sufficiently large number of points.
Specifically, as shown in FIGS. 6(a) and (b), in the case where a log A has a uniform contour (diameter=D, with a portion of the log having a concave portion A2 on its periphery), if a optimum yield axis is set at a position a in FIG. 6(a), for example, the largest number of continuous veneer sheets would be obtained from a portion of the log indicated at A3. However, in this case, many narrow veneer sheets with inappropriate widths (in a direction perpendicular to the direction of the fibers) would be produced at portions A4, thereby decreasing the yield of veneer sheets. On the other hand, if the optimum yield axis is set at a position b in FIG. 6(b), the quantity of continuous veneer sheets that can be obtained from a portion indicated at A5 would be smaller than in the example (a), while only a small number of veneer sheets with smaller width would be produced from a portion A6. Thus, in the case of FIG. 6(b), the yield of veneer sheet would be higher than in the example (a). Further, if the optimum yield axis is set at an intermediate position, which is not shown, between the positions of the aforementioned two examples, both the rate at which continuous veneer sheets can be obtained and the yield of veneer sheets would be somewhere between those of the two previous examples. Thus, the characteristics of the veneer sheets that are produced vary even in a log with a uniform contour, depending on at which position the optimum yield axis is set.
Thus, it is desirable to determine the position of optimum yield axis based on the size of the individual contours that are measured and their relative positional relationships, such that desired characteristics of veneer sheets can be obtained. In general, the optimum yield axis is most often set at a position such that a maximum number of continuous veneer sheets can be obtained, as shown in FIG. 6(a) However, the techniques whereby the optimum yield axis is set at the position (b) of FIG. 6 or the aforementioned intermediate position have also been put to practical use. Thus, the preferable position of optimum yield axis is not always constant even when the log has a uniform contour.
If the optimum yield axis is set at the position (b) in the example of FIG. 6, the maximum radius of rotation of the log would be D/2, whereas if the optimum yield axis is set at the position (a), the maximum radius of rotation would be larger than D/2. It will be seen, therefore, that the maximum radius of rotation of a log is dependent on the position of its optimum yield axis, namely the former cannot be determined unless the latter is set.
Now referring to FIG. 7, for the determination of the optimum yield axis of a log B, the log B is rotated about a preliminary axis c, and then the contour of the log B is measured by appropriate measuring devices C1 and C2 disposed in the vicinity of each end of the log B. By so doing, a minimum required condition for the measurement of the log can be satisfied, and in fact such measurement technique has been proposed and implemented. However, this type of measurement of the contour of the log is not capable of measuring the contour of the log at a convex portion B1 between measuring devices C1 and C2, which is formed by a branch or similar projection, so that it cannot determine the maximum radius of rotation of the log B simultaneously. If the knife carriage is set at a position that is determined on the basis of the result of measurement by the measuring devices C1 and C2, the convex portion B1 could possibly collide with the knife carriage. In other words, in order to determine the maximum radius of rotation of the log, the contour of the log must be measured at a sufficiently large number of positions along the axis of the log. In reality, the knife carriage is set at a standby position with some extra rearward distance provided so that the aforementioned problem can be avoided. This measure, however, reduces the availability factor of the veneer lathe.
In the case of the above-described manner of measurement, the contour of concave portions as well as convex portions that exist between the measuring devices cannot be measured. However, as far as the determination of the maximum radius of rotation is concerned, data concerning the contour of concave portions is not needed and its absence does not cause any problems. Specifically, there is no chance that a concave portion, if any, of the log would collide with the knife carriage of the lathe. Therefore, even if the contour data about the concave portion was not available, it does not pose any problem in determining the maximum radius of rotation of the log. This fact that the absence of data regarding the contour of any concave portions of the log does not pose any problem at least in determining the maximum radius of rotation is an exceptional matter in all types of measurement of the log, including the method and apparatus of centering a log according to the invention. For the determination of the optimum yield axis of the log suitable for the peeling thereof, it is still desirable to obtain appropriate contour data including that of concave portions.
A method of centering a log whereby the optimum yield axis suitable for the peeling of the log and the maximum radius of rotation can be determined together is proposed in JP Patent Publication (Kokai) No. 6-293002) entitled “Method and apparatus for centering and supplying a log”. According to this method, “a plurality of contour detectors are disposed on a log with substantially no gap provided therebetween along the length of the log, which is rotated once about a preliminary axis. Based on contour data obtained from at least two of the contour detectors, the optimum yield axis suitable for the peeling of the log is determined. Based on contour data provided by all of the contour detectors, the maximum radius of rotation of the log with respect to the optimum yield axis that has been determined is determined.”